I’ve been interested for a while in getting underfloor heating, probably of the hydronic variety. Underfloor heating offers some benefits in terms of less air movement and noise, and of giving a more liveable ambient heat – there is some evidence that the warmth of your feet is a key determinant of how comfortable your house is in winter.

The concept has been to use a large water storage tank as thermal mass, and to use evacuated tube water heating to heat that tank. This gives most of the benefit of having large thermal mass in your house, whilst giving control over when you take the heat out of that thermal mass.

This blog post explains why I think that’s probably not a great idea anymore, and what I’m thinking instead.

When reviewing the search terms that people visit here with (yes, I know, kind of like egosurfing), I saw a couple of people coming here looking for remote walls and thermal bridging. And I did recently do some searching and found relevant content.

To recap, remote wall construction is a technique where you leave your wall cavities empty, you sheath the outside of your timber framing, you run an air/moisture barrier over the sheathing, then you put polystyrene over the outside of that (continuous insulation), and finally cladding over all of it. The advantages are that:

your air barrier is moved outside the walls – so you don’t break your air barrier when you install power points and the like, so your house is much more air tight and therefore warmer

your insulation isn’t prone to thermal bridging from the studs

your insulation is rigid foam instead of batts – not as susceptible to collapsing and giving a lower R-value than you thought

You can combine with a similar system over the roof, giving you a conditioned ceiling cavity to run services in (and avoiding having fibreglass in your ceiling, which is one of my pet hates).

So, one of the questions that goes with all this is how you install continuous insulation, and cladding on the outside, without creating thermal bridging. The document I found that seems most authoritative is this study. And I think the conclusion is that the thermal bridging isn’t as bad as you might think, and in turn that means (thankfully) that you can just make your cladding as well attached as you need, and not mess around with fasteners that might not be durable or might not stand up to the wind you might expect.

So, I’ve been wondering for a while about whether you can mostly heat your house with a (large) solar hot water system, plus a good sized storage tank to bridge you through cold periods, plus a hydronic (under floor or radiator) system.

I’ve been researching some more today, and probably am not a lot closer to knowing. But what I have learned today I’ll include here, over time I’m going to post some more on this as I form more coherent ideas.

As part of thinking about building a house, I’ve given consideration to building a super insulated house. In different parts of the world this is called different things, often a passivhaus, named after the German standards.

Once you get reasonable insulation in your house, the main source of heat leakage is through air leakage – cold draughts in your house are a major source of energy loss. Air leakage is controlled through making the house air tight, which in turn relies on an air barrier. In most houses, this barrier is the dry wall / gyprock / gib board (depending on whether you’re American, Australian or kiwi). The trouble with this is that it’s really difficult to keep airtight – think of the opportunities for leakage:

I’m quite interested in getting a wood burner with a boiler (or wetback if you’re from NZ – which isn’t a racist term, but rather refers to the water running through a unit in the back of your wood burner).

I’ve looked at wood burners for a while, my observation is that those available in Australia and NZ are really not as good as those available in Europe. The typical burner down this end of the world is relatively inefficient, puts out a fair amount of particulate smoke (which are very bad for health).

In Europe, there are more interesting units. I have a lot of interest in this unit from Germany, which is relatively small in footprint, and pushes about 1.2kw into the room, 14.4kw into the water. You combine these with a water accumulator such as this unit, which stores hot water for a couple of days after the burner goes out. In theory you could run your domestic hot water and underfloor heating off a combination of these units plus a solar hot water unit – probably evacuated tubes – more about that in a later post.

Having said that, this thread is also interesting, indicating that Australian eucalyptus timber gives issues with resin buildup in a unit not built for it. It also suggests some NZ made units that could be interesting. These are more pure boilers than combined fireplace/boiler. Remember that my better half likes the look and feel of a fireplace in the house, so probably I need something that is a combined wood burner and boiler.

Another interesting point about the German units is that they are sealed – they don’t pull in air from the house, they take outside air into the combustion box, and then vent out the flue. This should reduce dust being dragged into your house, and also if you’re building a house that is very airtight for insulation reasons, avoid issues with lack of airflow and/or with dragging in cold air from outside.

If I ignore that fact that the better half wants a wood burner, I’m more than half tempted to just go with a heat pump instead, much lower maintenance, no cutting and carting of firewood, no ash to clean. But it’s not quite as heart warming in winter as a fire in the corner of the room.

I have a bit of an interest in energy usage. An online book that I read some time ago has just come to my attention again, without the hot air by David MacKay.

This book is interesting as it sidesteps all the discussions about climate change and energy use, and instead takes an engineering viewpoint of the question “could we replace fossil fuel usage with renewables?” The answers are interesting.

One of the things that takes us close to areas that interest me is the use of power in a house. I have been thinking about our next house, which I’m reasonably keen to build in a rural area. That would give us access to a range of alternatives for how we’d build the house, heat the house and power the house. One of the things I’m learning from my reading is that you shouldn’t really try to use one of everything – you end up with lots of small heating or power units that collectively achieve little, and you spend an awful lot of money putting it all together.

So, for example, you can heat with solar passive (windows), wood burners, solar thermal (hot water), heat pumps, gas, electric. It makes little sense to have one of each of these, it makes more sense to have one primary means that is “free” – so solar thermal for example, and one backup mechanism, say a wood burner or a heat pump. Whatever you use has to fit into your overall house plans – so if you’re going off grid with windmills or solar PV, then a heat pump probably isn’t a great idea. If you want cooling as well, or want to take advantage of seasonal storage of heat, then a heat pump may make great sense.

My inclination at the moment is to run underfloor heating using water (hydronic), primary heating coming from an oversized solar hot water system using evacuated tubes, and back that up with either a wood burner/boiler/wetback, or with a heat pump. Excess heat in summer is dumped into a swimming pool and/or into the ground. If a heat pump is in the plans, you can use a ground source heat pump to pull the heat you dumped into the ground in summer back out for heating in winter.

Heat pumps are attractive because they offer cooling, they’re super efficient, they’re very low maintenance, and they offer the opportunity for ground-sourced heat that leverages heat dumped in summer. Wood burners are attractive because they’d use local product (any useful farm should have spare trees to cut down), because the better half likes fire places, and because they work when the power’s off if you go off grid.

It’s not clear to me at the moment that it’d make sense to have both a wood burner and a heat pump, but I’d kind of like to.